1. Field of the Invention
The present invention relates generally to the field of optical imaging and more particularly to catadioptric optical systems used for microscopic imaging, inspection, and lithography applications.
2. Description of the Related Art
Many optical systems have the ability to inspect or image features on the surface of a specimen, such as inspecting defects on a semiconductor wafer or photomask, or alternately examining a biological specimen on a slide. Microscopes have been used in various imaging situations, including biology, metrology, semiconductor inspection, and other complex inspection applications where high resolution images of small areas and/or features are desired.
Many such imaging systems are currently available, including those disclosed by applicants in various other issued patents and patent applications, including but not limited to U.S. patent application Ser. No. 10/434,374 and U.S. Pat. No. 6,064,517. The objective disclosed in the '374 application is a small catadioptric objective exhibiting beneficial optical qualities. The small size catadioptric optical design of the '374 application exhibits generally good overall performance, especially at relatively high numerical apertures (NAs) over a very broad spectral range in the deep ultra-violet (DUV) light region. However, chromatic correction becomes increasingly difficult for performance in the DUV region. The objectives disclosed in the '517 patent are large catadioptric objectives that also exhibit desired optical qualities. The large catadioptric optical designs in the '517 patent are corrected over a narrow spectral bandwidth and are capable of ultra-high NAs imaging over very large field sizes.
These objectives generally provide advantages in their size, arrangement of lens groups, and the fact that they can be made from a single glass material. Such designs include lenses that can generally be categorized into three groups; a focusing lens group, a field lens group, and a catadioptric group. Lenses in these groups have very different arrangements and are used in different ways to achieve different goals.
Objectives in the aforementioned applications can benefit from enhanced NAs and field sizes. Generally, the NA represents the range of angles for which light can be delivered to and collected from a specimen using the design. The field size is the diameter at the specimen location over which all the optical performance requirements are satisfied. For example, certain systems may successfully and efficiently employ “ultra-high NA” imaging over large field sizes. Here, ultra-high NA illumination and imaging angles may include but are not limited to angles up to and above 76 degrees. Certain previous designs can only provide NAs lower than this ultra-high range, and exhibit smaller field size, larger optical element diameters, or very tight tolerances in the high end applications discussed. In general, larger NAs, larger field sizes, small element diameters, and loose tolerances for a given objective can be beneficial, particularly when constructed from a single material and/or operating in association with light energy at specific advantageous wavelengths. With respect to optical element diameters, smaller diameters result in more compact and less expensive to manufacture objectives. Further, smaller optical element diameters can be employed in certain specific applications, such as microscopy.
Based on the optical characteristics desirable in such a design, an ultra-high NA and large field can offer improved capabilities when inspecting specimens using light energy in the DUV range. For example, but not by way of limitation, operation at wavelengths of 193, 213, 244, 257, 266, 325, 355 or greater up through visible wavelengths may result in beneficial performance in certain circumstances. In addition, small size and loose manufacturing tolerances make the design practical to manufacture. Previous ultra-high NA designs do not demonstrate this capability
It would therefore be beneficial to provide a system and objective for use in conjunction with standard microscopes and microscopy applications that overcome the foregoing drawbacks present in previously known imaging systems. Further, it would be beneficial to provide an optical inspection system design having improved functionality over devices exhibiting the negative aspects described herein.